When it is desirable to combine many high-power input signals to achieve an extremely high-power output signal with very low-loss, 360-degree radial waveguide power combiners are typically employed. Such radial waveguide power combiners may typically incorporate waveguide or coaxial input port geometries. The input signals are then combined in the center of the radial waveguide by a transition from radial waveguide to a transverse electro-magnetic (TEM) mode coaxial transmission line. The coaxial transmission line is utilized because of its circular symmetry to achieve a high degree of amplitude and phase balance of the input signals for efficient combining. The coaxial transmission line must operate in its lowest order (TEM) mode in order to maintain amplitude and phase balance. This necessitates the cross-sectional dimensions of the coaxial transmission line to be below a threshold which depends upon the operating frequency range in order to render any undesirable higher order modes evanescent by decaying sufficiently to avoid perturbing the amplitude and phase balance. Invoking this cross-sectional dimension constraint on the coaxial transmission line limits both the peak and average power levels that can be achieved without component failure.